Wideband low distortion power amplifier

ABSTRACT

A circuit and apparatus for filtering harmful harmonics is disclosed. The circuit and apparatus include a power amplifier core that uses equally sized inverter based amplifiers. The amplifier core cells provide uniform load to all phases of a fundamental frequency to cancel all harmonics at an output. The power amplifier stages are driven into nonlinearity, and the combination of harmonics is performed at the output by varying series connected capacitors. The harmonic combination is performed at the output, leaving no further scope of nonlinearity in the signal chain.

TECHNICAL FIELD

This disclosure relates generally to the field of out-of-band harmonicfiltering and more specifically to harmonic filtering in communicationnetworks.

BACKGROUND

In electronic systems, input signals are processed through variouscomponents before they are available as an output signal as desired. Dueto various imperfections of electronic components, harmonics of theinput signal are generated at the output. Harmonics are manifested inoutput currents and voltages at integer multiples of the fundamentalfrequency of the input signal. Harmonics can cause signal distortionsand can lead to malfunctioning of electronic systems resulting indowntime and increased operating costs. In communication systemsespecially wireless communication systems, harmonics can distort dataresulting in incorrect data interpretation.

To eliminate harmonics, various harmonic filtering techniques are usedto filter out undesired harmonics and generate clean signal. Typically,harmonic filters are added towards the end of the circuit to capture or‘trap’ harmonics and filter undesired output signal at multiples of thefundamental frequency. These filters add extra cost for manufacturing ofelectronic systems and for high volume manufacturing, harmonic filtersbecome cost prohibitive.

In wireless communication, to communicate over long distance using acommunication device such as a cellular phone, high output poweramplifiers are needed; however, to comply with FCC regulations, strongout-of-band filtering is employed using external components in thecommunication devices for impedance conversion. Traditional solutionsrealize high efficiency transmitters by driving transmitters near railwaveforms and use many external components to filter out harmonics ofthe fundamental tone. This solution typically requires a high voltagetransistor to sustain high waveform swing and many externals componentsto filter out harmonics which increases the losses. These externalcomponents are highly undesirable in the communication device becausethey add cost and use up significant battery power. Ideally, electronicsystems need to be designed such to avoid undesired harmonics in theoutput signal and reduce the cost of products.

SUMMARY

In accordance with an embodiment, a circuit is disclosed. The circuitincludes a frequency oscillator for generating a fundamental signalfrequency, a frequency divider coupled to the frequency oscillator andconfigured to divide the fundamental signal frequency to generate one ormore phases of the fundamental signal frequency, a power amplifiercoupled to the frequency divider and configured to, receive thefundamental signal frequency and the one or more phases, and cancelharmonic frequencies of the fundamental signal frequency.

In accordance with another embodiment, an apparatus is disclosed. Theapparatus includes a frequency oscillator for generating a signalfrequency, a frequency divider coupled to the frequency oscillator andconfigured to divide the signal frequency to generate one or moreequally spaced phases of the signal frequency, a power amplifier coupledto the frequency divider and configured to, receive the one or morephases of the signal frequency, and cancel harmonics of the signalfrequency, wherein one or more of the frequency oscillator and thefrequency divider are programmable to generate a plurality of signalfrequencies and a plurality of phases at a fundamental frequency of thesignal frequency respectively.

In accordance with yet another embodiment, a device is disclosed. Thedevice includes a transceiver, and a processing unit coupled to thetransceiver, wherein the transceiver is configured to receive areference phasor signal at a fundamental signal frequency and one ormore equally spaced phasors at the fundamental signal frequency, provideuniform load to each one of the phasor at the fundamental signalfrequency and cancel the one or more harmonic frequencies of thefundamental signal frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary system for managing harmonics accordingto an embodiment.

FIG. 2 illustrates an exemplary system with implementation of widebandlow distortion power amplifiers according to another embodiment.

FIG. 3 illustrates an exemplary implementation system fordigital-to-analog converters (DAC) according to another embodiment.

FIG. 4 illustrates various exemplary multi-band implementations ofwideband low distortion power amplifier according to another embodiment.

FIG. 5 illustrates an exemplary output waveform generated using thewideband low distortion power amplifier according to another embodiment.

FIG. 6 illustrates an exemplary wireless network using the wideband lowdistortion power amplifier according to another embodiment.

DETAILED DESCRIPTION

The following description provides many different embodiments, orexamples, for implementing different features of the subject matter.These descriptions are merely for illustrative purposes and do not limitthe scope of the invention.

Referring to FIG. 1, an exemplary system 100 for managing harmonics isillustrated according to an embodiment. System 100 includes inverterbased wideband lower distortion power amplifiers 110-150. For purposesof simplification, five power amplifiers are illustrated; however, thesystem in not in any way limited to the illustrated number of poweramplifiers. Depending on the number of harmonics to be processed, thenumber of power amplifiers can be increased or decreased accordingly.Power amplifier 130 receives the input signal 160 with a phase Φ₀ of afundamental frequency f₀ of input signal 160. Power amplifiers 110 and120 receive signals with phases Φ⁻¹ and Φ⁻² of the fundamental frequencyf₀ respectively. Similarly, power amplifiers 140 and 150 receive signalswith harmonic frequencies Φ₁ and Φ₂ of the fundamental frequency f₀respectively. The phase Φ₁ is complement of the phase Φ⁻¹ in such thatwhen they are subjected to a uniform load and then combined, they cancelall the harmonics. Similarly Φ₂ is complement of Φ⁻² and so on.

Each power amplifier is coupled to an output capacitor 170 a-e forweighting respective phasor components. For example, power amplifier 130is coupled to capacitor 170 c with capacitance C₀, power amplifiers 110and 120 are coupled to capacitors 170 a and 170 b with capacitance βC₀and αC₀ respectively. Similarly, power amplifiers 140 and 150 arecoupled to capacitors 170 d and 170 e with capacitance αC₀ and βC₀respectively. The combined output of capacitors 170 a-e is thenprocessed by the matching network 160 before transmitting the signal viaantenna 170. Matching network is formed using lossless reactivecomponents (i.e. inductors, capacitors, or the like) to transformrequired impedance for delivering optimum power to the antennaimpedance. In an embodiment, the typical impedance of the antenna isabout 50Ω. The output of capacitors 170 are combined thus harmonics ofthe input signal 160 are canceled before the signal is processed by thematching network and thus the signal does not require additionalharmonic filtering prior the transmission, and other matching networksmaybe utilized along with antennas that are relatively insensitive tothe variation in the surrounding environment.

The core of power amplifiers 110-150 uses equally sized inverter/buffertype limiting amplifiers providing equal load per phase of thefundamental carrier frequency and it is driven by N phases of thefundamental carrier. The pull-up (PMOS) and pull-down (NMOS) transistorsin each of the inverter/buffer type stages utilize similar or same widthand length dimensions, which allows them to provide equal load per phaseof the fundamental carrier frequency. In the exemplary illustratedembodiment, N=5; however, as explained hereinabove, the number of poweramplifier stages can be based on the number of harmonics to beprocessed, and may include an even number of stages as well. The corecells of power amplifiers 110-150 provide substantially constant load toall phases of the fundamental frequency of the input signal so thatharmonics of the fundamental frequency are cancelled at the output. Theload of the power amplifiers 110-150 (inductive, or capacitive) can bedesigned such that each phase of the fundamental frequency is subject touniform output load and thus the harmonics are cancelled out at theoutput when all phases are combined. For example, if the reference phaseΦ₀ of the fundamental frequency f₀ and the two equally spaced phases inthe opposite direction Φ₁ and Φ⁻¹ of the fundamental frequency f₀ arecombined with respect to a pre-determined scaling factor, then theyconstructively combine energy at the fundamental frequency, anddestructively nullify energy at the harmonic frequencies leaving a cleanoutput signal at the fundamental frequency f₀.

Referring to FIG. 2, an exemplary system 200 with implementation ofwideband low distortion power amplifiers is illustrated according toanother embodiment. The system 200 includes power amplifiers 210 a-n.The number ‘n’ can be selected based on the design choice for harmoniccancellation for example, if the design choice is to cancel two positiveand two negative harmonics, the system 200 will include five poweramplifiers including one for the fundamental frequency. The ‘n’ phasesare derived from the input signal working at ‘n’ times the fundamentalfrequency. Each phase signal is first divided by divider elements D1-nand then is buffered by buffers BUF1-n. The buffered signals are thenprocessed by digital to analog converters DAC1-n. The buffers BUF1-n aredesigned to isolate DACs and the divider stages and typically containmore than one stage of inverters or buffers. In the present example,input signals are fully differential and each signal phase sees the sameloading to DACs. Further, in the present example, the DAC stages areweighted to provide harmonic nulls at (2n+1) times the center frequencyof the fundamental carrier; however, any other weighting scheme can beused based on any particular given implementation. In the exemplaryembodiment, odd number of stages is illustrated; however, an even numberof stages may also be used along with the programmability.

The power amplifier stages are driven into nonlinearity and thecombination of signals is performed at the output by varying the valueof series connected reactive elements (in this specific example,capacitors). When ‘n’ is increased, significant harmonic suppression canbe obtained, thereby eliminating the need of external filtering networksand harmonic traps. Because the harmonic combination occurs at theoutput, this leaves no further nonlinearity in the signal chain.Optionally, to reject higher order harmonics that are not addressed bythe power amplifier stages, harmonic traps may be used at the output.The power amplifier stages and drivers use inverter based topology,tapered from the divider to the driver and are scaled from each other.This approach offers both, high linearity and high efficiency using thesame architecture. Further, it can offer a wide band operation bychanging the center frequency of the signal by changing the dividerratio. This also eliminates need for external components for harmonicfiltering thus reducing the product cost.

Referring to FIG. 3, another exemplary implementation system 300 fordigital-to-analog converters (DAC) is illustrated according to anotherembodiment. In the exemplary implementation, system 300 includes fiveDACs 310A-E along with respective inverter/buffer pair 305A-E forpositive cycle of an incoming signal for example, a radio frequency (RF)signal, and five DACs 320A-E along with respective inverter/buffer pair315A-E for negative cycle of the RF signal. In the exemplary embodiment,the DACs are illustrated as capacitors along with respectiveinverters/buffers; however, any other reactive components can be used asDACs with appropriate inverters/buffers. Typically, in harmoniccancellation application, DACs are weighted with respect to apre-determined sine/cosine waveform and are different from traditionalbinary or thermometer weighting. In another embodiment, the combinationof inverter/buffer and capacitor can be used as a “hybrid-weighted DAC”where the active elements (transistor based inverter/drivers) areequally weighted, as explained hereinabove, and the reactive elements(capacitors/inductors) are weighted according to trigonometric function.

For explanation purposes, five pairs of DACs with inverter/buffers areshown; however, the number of DACs can be any number ‘N’ according toany design implementation choice based on the number and/or type ofharmonics to be canceled. Each one of the inverter/buffer pair 305A-Eand 315A-E is powered by a power supply VDD and is coupled tocorresponding DAC 310A-E and 320A-E with capacitance C, αC, and PC asillustrated similar to ones in FIG. 1. Each one of the DACs 310A-Ereceive selected phase of the input RF signal for example, DAC 310Creceives the positive cycle of the fundamental frequency or tone of theRF signal, DACs 310A-B receive the first and second harmonics of thepositive cycle that are prior to the fundamental frequency of the RFsignal respectively, and DACs 310D-E receive the first and secondharmonics of the positive cycle that are after the fundamental frequencyof the RF signal respectively. Similarly, DACs 320A-E receive thenegative cycles of the fundamental frequency and its correspondingharmonics of the input RF signal.

In the exemplary implementation, DACs 310 and 320 are illustrated usingthe transistor elements; however, one skilled in the art will appreciatethat DACs 310 and 320 can also be implemented using other passivecomponents such as for example, an inductive element. An inductive coilwith various taps can be used as DACs 310 and 320 to select thefundamental frequency and its harmonics. DACs 310 and 320 can beconfigured using passive components such as, inductors, and/orcapacitors and therefore they can also be implemented as part of thematching networks 160 or 250 as illustrated in FIGS. 1&2. Further,because DAC components are passive, they provide exceptional phaseaccuracy and do not cause any harmonic degradation. Passive DACs 310 and320 use low-loss reactive elements, and can be configured to withstandhigh voltage or current handling capabilities, leading to high outputpower. This enables the scalability of the proposed technique, and theefficiency of a system using the DAC system 300 is maximized andscalable, both at the same time. Further, this enables reconfigurabilityof the proposed scheme for multiple wireless standards.

The power supply VDD for each one of the buffer/inverter 305A-E and315A-E can be implemented as variable power supply to compensate for anymismatch in the output of the system for example, if some of theharmonics of the fundamental frequency are ‘leaked’ into output, then atinitial calibration stage, the VDD for respective harmonics can beadjusted to ensure that the particular harmonics is completely canceledat the output. One skilled in the art can appreciate that the powersupply VDD can be adjusted using various component combinations such asvariable resistor network or the like implementation to adjust the inputpower supply as desired.

Typically, harmonics of the fundamental frequency are measured bykeeping a frequency synthesizer (not shown) at the same frequency as ofthe system for example, if the system is a transmitting unit, then thefrequency synthesizer can have the same frequency as the transmitter(TX). For an implementation using six phase cancellation, a 6-stagedelay can be used to generate all six phases of the fundamentalfrequency. Similarly, an ‘N’ stage delay can be used to generate ‘N’phases of the fundamental frequency for any given implementation. In theexemplary embodiment, multiple clock phases are used at low supplyvoltage, thereby making it suitable for integration with low supplyvoltage DC-DC converters in any particular implementation. Further, theexemplary implementation uses low voltage transistors, which allows alower power implementation in any system. Using the exemplaryimplementation, the entire harmonic filtering can be done inside a chipthus avoiding the use of off-chip additional harmonic traps andfiltering using external components. The exemplary implementation leadsto a low-cost solution and it can also be implemented in a system withlow quality factor of antennas such as the ones typically used inautomotive and similar applications.

Referring to FIG. 4, various exemplary multi-band implementations 400 ofwideband low distortion power amplifier are illustrated according to anembodiment. Implementation 410 includes a local oscillator 412, adivider network 414, a power amplifier 416, and a matching network 418.In this exemplary implementation, the local oscillator 412 is variableand configured to tune to multiple frequencies to cover wide range offrequencies. The divider network 414 can be any network configured todivide the local oscillator 412 frequency to the desired number ‘N’ ofharmonics. The power amplifier 416 and matching network 418 can besimilar to the ones illustrated in FIGS. 1&2.

Implementation 430 is another illustration of the application of poweramplifier for harmonic filtering according to another embodiment.Implementation 430 includes a local oscillator 432, a divider network434, a power amplifier 436, and a matching network 438. In thisparticular implementation, the local oscillator 432 is configured toprovide a fixed specific center frequency. The divider network 434 isprogrammable by enabling or disabling the ring of delay stages thuscreating multiple phases of the center frequency. For example, an oddinteger divider by (2n+1) can be created by connecting (2n+1) equaldelay stages and programmed to (2n−1) by shorting out two of the stagesof the delay cell ring. As stated hereinabove, an even number of stagesmay also be used in addition to the reconfigurability as well. The poweramplifier 416 and matching network 418 can be similar to the onesillustrated in FIGS. 1&2.

Implementation 450 is yet another illustration of the application ofpower amplifier for harmonic filtering according to another embodiment.Implementation 450 includes a local oscillator 452, two divider networks454 and 455, two power amplifiers 456 and 457, and a matching network458. In implementation 450, input signal such as for example a RF signalfrom the local oscillator 452 is provided to both fixed divider networks454 and 454. Each divider network 454 and 455 interfaces withcorresponding power amplifier 456 and 457 respectively. Outputs frompower amplifiers 456 and 457 are combined together in the matchingnetwork 458 similar to as illustrated in FIGS. 1&2. In thisimplementation, one chain may be active at a particular given time. Thisimplementation provides a wide range of frequency coverage with the useof multiple divider networks. For explanation, in this exemplaryillustration only two divider networks are shown; however, multipledivider networks can be used based on the design implementation choiceand the range of frequencies to be covered.

Referring to FIG. 5, an exemplary output waveform 500 generated usingthe wideband low distortion power amplifier is illustrated according toan embodiment. The waveform 500 is generated using N=5 implementationwith five power amplifiers similar to the ones illustrated in FIGS. 1&2.In this particular implementation, five phases of the fundamentalfrequency are used to generate the near sinusoidal output waveform 500.The waveform 500 is generated using a reference phase Φ₀ of fundamentalfrequency f₀ and four other phases Φ⁻¹, Φ⁻², Φ₁, and Φ₂. A weighted sumof five phasors leads to the formation of a sinusoidal wave shape 500.In the present example, the number of phasors is N=5; however, as statedhereinabove, any number phasors or harmonics ‘N’ can be used based onany particular given implementation. A higher ‘N’ further smoothens thesinusoidal waveform shape 500 while increasing the power use andadditional components in the circuit.

Referring to FIG. 6, an exemplary wireless network 600 is illustratedaccording to another embodiment. Network 600 includes a network element610. The network element 610 can be any wireless communication networkelement for example, a base station, an access point, a network relay, anetwork extender, a wireless router, or any other device capable ofconnecting to a network and provide wireless communication connectionsto various devices. The network 600 further includes a wirelesscommunication device 602. The wireless communication device 620 can beany device such as for example, a cell phone, a laptop computer, apersonal digital assistant device (PDA), or the like. The network 600can also include many other devices capable of wirelessly communicatingwith the network element 610 such as control systems, printers, consumerelectronic devices, and various other devices and systems.

The wireless communication device 620 includes various system componentssuch as a transceiver 622, a processor 624, storage devices 626, and anantenna 628. The device 620 can include various other components likedisplays, keyboards, multiple antennas, and the like (not shown). Thenetwork 600 can be based on any single or combination of protocols suchas cellular, WiFi, Bluetooth, and various other wireless communicationprotocols. When wireless devices such as device 620 has data tocommunicate, then the wireless device 620 processes the data andmodulates it into a carrier frequency that its transceiver uses totransmit data to the network element 610 or to any other network devicein the network 600.

When the wireless device 620 processes the data and uses a particularfrequency to modulate and transmit data, it may encounter challenges ofharmonics of the carrier frequency, which may impact the performance ofthe wireless device transmission and regulatory compliance. The wirelessdevice 620 may include a wideband low distortion power amplifierscircuit illustrated in FIGS. 1&2 to manage the carrier harmonics andtransmit a clean signal based only on the designated carrier frequencyfor transmission. Using the wideband low distortion power amplifierscircuit according to an embodiment allows the wireless device totransmit data with minimum distortion and significantly mitigates theharmonic problem in the transmission.

The foregoing outlines features of several embodiments so that those ofordinary skill in the art may better understand various aspects of thepresent disclosure. Those of ordinary skill in the art should appreciatethat they may readily use the present disclosure as a basis fordesigning or modifying other processes and structures for carrying outthe same purposes and/or achieving the same advantages of variousembodiments introduced herein. Those of ordinary skill in the art shouldalso realize that such equivalent constructions do not depart from thespirit and scope of the present disclosure, and that they may makevarious changes, substitutions, and alterations herein without departingfrom the spirit and scope of the present disclosure.

Although the subject matter has been described in language specific tostructural features or methodological acts, it is to be understood thatthe subject matter of the appended claims is not necessarily limited tothe specific features or acts described above. Rather, the specificfeatures and acts described above are disclosed as example forms ofimplementing at least some of the claims. Various operations ofembodiments are provided herein. The order in which some or all of theoperations are described should not be construed to imply that theseoperations are necessarily order dependent. Alternative ordering will beappreciated having the benefit of this description. Further, it will beunderstood that not all operations are necessarily present in eachembodiment provided herein. Also, it will be understood that not alloperations are necessary in some embodiments.

Moreover, “exemplary” is used herein to mean serving as an example,instance, illustration, etc., and not necessarily as advantageous. Also,although the disclosure has been shown and described with respect to oneor more implementations, equivalent alterations and modifications willoccur to others of ordinary skill in the art based upon a reading andunderstanding of this specification and the annexed drawings. Thedisclosure comprises all such modifications and alterations and islimited only by the scope of the following claims. In particular regardto the various functions performed by the above described components(e.g., elements, resources, etc.), the terms used to describe suchcomponents are intended to correspond, unless otherwise indicated, toany component which performs the specified function of the describedcomponent (e.g., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure. In addition, while aparticular feature of the disclosure may have been disclosed withrespect to only one of several implementations, such feature may becombined with one or more other features of the other implementations asmay be desired and advantageous for any given or particular application.

What is claimed is:
 1. A circuit comprising: a frequency oscillator forgenerating a fundamental signal frequency; a frequency divider coupledto the frequency oscillator and configured to divide the fundamentalsignal frequency to generate one or more phases of the fundamentalsignal frequency; a power amplifier coupled to the frequency divider andconfigured to, receive the fundamental signal frequency and the one ormore phases, and cancel harmonic frequencies of the fundamental signalfrequency.
 2. The circuit of claim 1, wherein the power amplifier isfurther configured to provide uniform output load to the fundamentalsignal frequency and the one or more harmonic frequencies.
 3. Thecircuit of claim 1, wherein the power amplifier comprising: one or morebuffers configured to buffer one or more of the fundamental signalfrequency and the harmonic frequencies; and one or moredigital-to-analog converters (DAC) coupled to the one or more buffersand configured to combine the fundamental signal frequency and the oneor more phases of the fundamental signal frequency, and cancel theharmonic frequencies of the fundamental signal frequency
 4. The circuitof claim 3, wherein the DACs are configured using passive components. 5.The circuit of claim 3, wherein the DACs are further configured toprovide a uniform output load to each one of the phases of thefundamental signal frequency.
 6. The circuit of claim 5, wherein eachone of the buffers is coupled to a corresponding power supply.
 7. Thecircuit of claim 6, wherein the power supply is a variable power supplyand configured to one or more of change power supplied to correspondingDAC to adjust the output load of the DAC, change output power, andadjust a level of the harmonic cancellation.
 8. An apparatus comprising:a frequency oscillator for generating a signal frequency; a frequencydivider coupled to the frequency oscillator and configured to divide thesignal frequency to generate one or more equally spaced phases of thesignal frequency; a power amplifier coupled to the frequency divider andconfigured to, receive the one or more phases of the signal frequency,and cancel harmonics of the signal frequency, wherein one or more of thefrequency oscillator and the frequency divider are programmable togenerate a plurality of signal frequencies and a plurality of phases ata fundamental frequency of the signal frequency respectively.
 9. Theapparatus of claim 8, wherein the frequency divider is a programmablefrequency divider and the apparatus further comprising: a plurality ofpower amplifiers configured to receive the plurality of the phases atthe fundamental frequency, provide a uniform load to each one of the ofthe harmonics of the signal frequency, cancel each one of the harmonicsof the signal frequency, and amplify signal at the fundamentalfrequency.
 10. The apparatus of claim 8, wherein the power amplifiercomprising: a buffer configured to buffer the one or more phases of thesignal frequency; a digital-to-analog converters (DAC) coupled to thebuffer and configured to convert a digital representation of the one ormore of the phases and the harmonics of the signal frequency intocorresponding analog frequencies; and a capacitor coupled to the DACsand configured to filter the analog frequencies.
 11. The apparatus ofclaim 10, wherein the DACs are further configured to provide a uniformload to each one of the plurality of phases at the fundamentalfrequency.
 12. The apparatus of claim 11, where in the power amplifieris coupled to a power supply.
 13. The apparatus of claim 12, wherein thepower supply is a variable power supply and configured to one or moreof: change power supplied to corresponding DAC to adjust the output loadof the DAC, change output power, and adjust a level of the harmoniccancellation.
 14. A device comprising: a transceiver, and a processingunit coupled to the transceiver, wherein the transceiver is configuredto: receive a reference phasor signal at a fundamental signal frequencyand one or more equally spaced phasors at the fundamental signalfrequency, provide uniform load to each one of the phasor at thefundamental signal frequency and cancel the one or more harmonicfrequencies of the fundamental signal frequency.
 15. The device of claim14, wherein the transceiver further comprising: a power amplifierincluding: one or more buffers configured to buffer one or more of theequally spaced phasors at the fundamental signal frequency; and one ormore digital-to-analog converters (DAC) coupled to the one or morebuffers and configured to convert a digital representation of the one ormore of the phasors at fundamental signal frequency into a correspondingoutput analog signal without harmonics.
 16. The device of claim 15,wherein the DACs are further configured to provide uniform output loadto one of the fundamental signal frequency and the one or more harmonicfrequencies.
 17. The device of claim 15, where in each one of the one ormore DACs is coupled to a corresponding power supply.
 18. The device ofclaim 17, wherein the power supply is a variable power supply andconfigured to one or more of change power supplied to corresponding DACto adjust the output load of the DAC, change an output power, and adjusta level of the harmonic cancellation.